XnView can display all *.ICO files which conform the icon format of Windows®.
In some cases, icons are displayed as a black block. This is no bug of XnView, the reason is that the icon file contains two or more icons. In this case, look in the toolbar and there will be two new buttons, namely Previous Frame/Next Frame. In addition, the number of the current frame and the number of frames appear in the status bar. Use the buttons in the tool bar to display the icons which are contained in the icon file.
The icons with 256 colours created with the software Ax-Icons ® of the company Axialis™ have two frames.
Icons which do not follow the Windows icon format are shown as black in XnView. Sometimes these icons can be displayed using the file manager Total Commander® .
Here is an example of an Icon with 3 frames, the second frame is currently displayed: